The proposed former isolation of the basin of Cuatro Ciénegas is supported by evidence found in studies of other turtles in the basin. Of the four kinds of turtles known to occur there (_Terrapene coahuila_, _P. s. taylori_, _Trionyx spinifer emoryi_, and _Trionyx ater_), all but _T. spinifer_ seem to be endemic. These three kinds comprise a graded series, in regard to their degree of differentiation from closest known relatives, as follows: 1) _Terrapene coahuila_ is morphologically the most generalized and primitive of living box turtles; the species is unique in its highly aquatic mode of life (see Legler, 1960:532-534, for brief discussion of relationships within genus Terrapene); 2) _Trionyx ater_ seems to represent a relict population of pre-_Trionyx spinifer_ stock; presumably, _spinifer_ has reinvaded the basin of Cuatro Ciénegas in relatively recent times and, as noted above, _spinifer_ and _ater_ now occur sympatrically (at least in a geographic sense) in the basin (Webb and Legler, _op. cit._); and, 3) evidence presented above suggests that _P. s. taylori_ intergrades with _P. s. elegans_ outside the basin. The three endemic populations of turtles at Cuatro Ciénegas therefore, differ by varying degrees from their closest living relatives. This variation in degree of difference possibly results from varying periods of isolation. Probably the basin of Cuatro Ciénegas has been isolated from, and reconnected to, the lower Río Grande drainage at several times in the past. The relationships of fishes in the basin, now under study by other workers, also suggest that the basin was isolated more than once. _Remarks._--Local names for the above-mentioned localities in the basin of Cuatro Ciénegas are as follows: Anteojo (6 mi. W Cuatro Ciénegas); El Mojarral (8.5 mi. SW); and Ojo de Agua de Tío Candido, on Rancho Orozco (16 km. S). The Río Chiquito is referred to by some natives as "Río Colorado" and by some as "Río Salado." The local name for _P. s. taylori_ is _tortuga negra_ (the name is used also for _Terrapene coahuila_).